The use of interferometric methods to inspect the surface of an object for defects or to measure the relief of an object is well known. Generally stated, these methods consist in generating an interferometric pattern on the surface of the object and then analyzing the resulting interferometric image (or interferogram) to obtain the relief of the object. The interferometric image generally includes a series of black and white fringes.
Interferometric methods that require the use of a laser to generate the interferometric pattern are called “classic interferometric methods”. In such classic methods, the wavelength of the laser and the configuration of the measuring assembly generally determine the period of the resulting interferogram. Classic interferometry methods are generally used in the visible spectrum to measure height variations in the order of micron.
However, it has been found difficult to use such method to measure height variations (relief on a surface showing variations beyond 0.5–1 μm when they are implemented in the visible spectrum. Indeed, the density of the black and white fringes of the resulting interferogram increases, causing its analysis to be tedious.
Another drawback of classic interferometric methods is that they require measuring assemblies that are particularly sensitive to noise and vibrations.
Surface inspection methods based on Moiré interferometry allow measuring the relief of an object in the visible spectrum with accuracy much more than the accuracy of classic interferometric methods. These methods are based on the analysis of the frequency beats obtained between 1) a grid positioned over the object to be measured and its shadow on the object (“Shadow Moiré Techniques”) or 2) the projection of a grid on the object and another grid positioned between the object and the camera that is used to take a picture of the resulting interferogram (“Projected Moiré Techniques”). In both cases, the frequency beats between two grids produce the fringes of the resulting interferogram.
More specifically, the Shadow Moiré technique includes the steps of positioning a grid near the object to be measured, providing illumination from a first angle from the plane of the object (for example 45 degrees) and using a camera, positioned at a second angle (for example 90 degrees from the plane of the object), to take pictures of the interferogram.
Since the distance between the grid and the object varies, this variation of height produces a variation in the pattern of the interferogram. This variation in the pattern can then be analysed to obtain the relief of the object.
A drawback to the use of a Shadow Moiré technique for measuring the relief of an object is that the grid must be positioned very close to the object in order to yield accurate results, causing restrictions in the set-up of the measuring assembly.
The Projected Moiré technique is very similar to the Shadow Moiré technique since the grid, positioned between the camera and the object, has a function similar to the shadow of the grid in the Shadow Moiré technique. However, a drawback of the Projected Moiré technique is that it involves many adjustments and therefore creates more risk of inaccuracy in the results since it requires the positioning and tracking of two grids. Furthermore, the second grid tend to obscure the camera, preventing it from being used simultaneously to take other measurements.
A method and a system to measure the relief of an object free of the above-mentioned drawbacks of the prior-art are thus desirable.